Enzymatic activities associated with decomposition of particulate organic matter in two shallow ponds

Particulate organic matter (POM) is an important source of energy in aquatic systems, where decomposition rates have traditionally been studied using the litter-bag technique. However, this method has limitations for estimating in situ decomposition rates, especially for fine particles. In this study the litter-bag technique is combined with measurements of microbial enzyme activities to estimate POM mass loss rates in situ. Litter-bags containing benthic POM in two size ranges, coarse (>1 mm) and fine (0.063–0.5 mm), were placed in two contrasting ponds (Toro and Oro) located in the Doñana area in southern Spain. Litter-bags were collected over a period of a year and analysed for mass loss and the activities of five extracellular enzymes (β-glucosidase, β-xylosidase, β-N-acetylglucosamidase, phenol oxidase, alkaline phosphatase). In situ sediment POM samples were also collected, sorted in the same size ranges, and assayed for the same enzyme activities. Using separate regression models relating mass loss to average cumulative enzyme activity, the mean turnover times for in situ coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM) were 149 and 2778 days in Toro pond and 382 and 4347 days in Oro pond, respectively. This compared with turnover times of 578 and 2109 days for Toro and 778 and 5296 days in Oro pond for POM confined in bags. Integrating enzyme measurements and litter-bag techniques provides in situ organic matter turnover rates and may allow the assessment of global controlling factors (e.g. flooding regime, temperature and POM quality) in the decomposition process in temporally and spatially heterogeneous environments.